This invention relates in general to xerography and more specifically to a novel photoreceptor and process for preparing and using the photoreceptor.
Vitreous and amorphous selenium photoconductive materials have enjoyed wide use in reusable photoconductors in commercial xerography. However, the spectral response of these materials is limited largely to the blue-green portion of the visible spectrum, i.e. below 5200 Angstrom units.
Selenium also exists in a crystalline form known as trigonal or hexagonal selenium. Trigonal selenium is well known in the semiconductor art for use in the manufacture of selenium rectifiers.
In the past, trigonal selenium was not normally used in xerography as a photoconductive layer because of its relatively high electrical conductivity in the dark, although in some instances, trigonal selenium can be used in a binder configuration in which the trigonal selenium particles are dispersed in a matrix of another material such as an electrically active organic material or vitreous selenium.
It is also known that a thin layer of trigonal selenium overcoated with a relatively thick layer of electrically active organic material, forms a useful composite photosensitive member which exhibits improved spectral response and increased sensitivity over conventional vitreous selenium-type photoreceptors. This device and method are described, for example, in U.S. Pat. No. 3,961,953 to Millonzi et al.
It is also known that when using trigonal selenium, whether it be dispersed in a binder or used as a generation material in a composite photoconductor device, the trigonal selenium exhibits a high dark decay after the photoreceptor has been cycled in a xerographic process. This is referred to as fatigued dark decay. Also, after cycling the photoreceptor in a xerographic process, the photoreceptor will not accept as much charge as it did initially. Fatigued dark decay is the dark decay observed after a photoreceptor has completed at least one xerographic cycle, is erased and recharged.
A process for controlling dark decay by treatment of trigonal selenium is described in U.S. Pat. No. 4,232,102 to Horgan et al. The process provides treated trigonal selenium for photosensitive devices with improved cyclic charge acceptance and control and also improved dark decay both initially and after cycling an imaging member in a xerographic process. The treatment process involves, for example, swirling washed trigonal selenium in a 0.6 normal (N) solution of sodium hydroxide for one-half hour and then allowing the solids to settle out and remain in contact with the sodium hydroxide solution for 18 hours. The supernatent liquid is decanted and retained and the treated trigonal selenium is filtered with filter paper. The retained supernatent liquid is used to rinse the breaker and funnel. The trigonal selenium is then dried at 60.degree. C. in a forced air oven for 18 hours. The total sodium selenite and sodium carbonate levels in the resulting mixture average approximately 1.0 percent by weight on an approximately equimolar basis based on the weight of the trigonal selenium.
Although very good results may be achieved with the specific process described in U.S. Pat. No. 4,232,102, the 18 hours utilized for contacting the trigonal selenium with sodium hydroxide and the 18 hours employed for forced air drying are time consuming. Moreover, the sodium content of the final treated trigonal selenium cannot be accurately predicted and the sodium content of the final treated trigonal selenium can vary as much as 52% from the lowest weight percent sodium content to the highest weight percent sodium content Further, undesirable absorption of water occurs during storage prior to incorporation of the sodium doped selenium into a photosensitive device.
Another process for controlling dark decay by treatment of trigonal selenium is described in copending U.S. patent application Ser. No. 557,498, entitled "PROCESS", filed Dec. 1, 1983 in which an electrostatographic photosensitive device is prepared by combining a sodium additive comprising sodium carbonate, sodium bicarbonate, sodium selenite, sodium hydroxide or mixtures thereof with trigonal selenium particles, an organic resin binder and a solvent for the binder to form a milling mixture, milling the milling mixture to form a uniform dispersion and applying the dispersion to a substrate in an even layer and drying the layer. The sodium additive may be added to form the milling mixture as an anhydrous salt or in a concentrated aqueous solution. If the sodium additive is in the form of a concentrated aqueous solution, it should contribute to the milling mixture less than about 20 percent by weight water based on the total weight of the trigonal selenium. The milling mixture is milled until a uniform dispersion of trigonal selenium particles having an average particle size of between about 0.01 micrometer and about 5 micrometers is formed.
Although very good results may be achieved with the specific process described in the aforesaid copending U.S. patent application Ser. No. 557,498, the process requires the presence of a sodium compound which can eventually cause the pitting and eventual loss of metal ground planes such as aluminum ground planes when the photoreceptor is cycled many thousands of cycles under high huimidity conditions. Pitting can result in black spots in the background areas of copies and loss of the ground plane is manifested by an inability of the photoreceptor to form a toner image.